1. Field of the Invention
The present invention relates to a ground relay system in a multiplex direct grounding system which is capable of appropriately detecting a grounding failure caused by a load imbalance of a multiplex direct grounding system distribution.
2. Description of the Related Art
A conventional ground relay system in a multiplex direct rounding system will be described with reference to the accompanying drawings. FIG. 11 is a circuit diagram showing a conventional relay system in a multiplex direct grounding system in a high-voltage distribution system.
Referring to FIG. 11, reference numeral 1 denotes a distribution main transformer primary side winding, 2 is a distribution main transformer secondary side wiring, 3 is a transformer secondary side circuit breaker, 4 is a main transformer secondary side bus, 5 is a neutral point ground line at a main transformer secondary side, 6 is a CT (current transformer) attached to the neutral point ground line 5, 7 is a ground over-current relay that is connected to a CT secondary side circuit, 8 is a main transformer secondary side CT attached to the main transformer secondary side, 9 is an over-current relay attached to the secondary side of the CT 8, 10 is a PT (transformer) attached to the main transformer secondary side bus 4, and 11 is a relay for an overvoltage or undervoltage.
Also, in the figure, reference numerals 100 and 200 denote circuit breakers of feeders F10 and F20, respectively, 101 and 201 are CTs (current transformers) located on the feeders F10 and F20, respectively, 301 and 302 are protection relays made up of an over-current relay and a ground over-current relay, 102 and 202 are first sections, 10-1 and 20-1 are second section switches, 103 and 203 are second sections, 10-2 and 20-2 are third section switches, 104 and 204 are third sections, 10-3 and 20-3 are fourth section switches, 105 and 205 are fourth sections, and 40 is a switch that associates the feeder F10 with the feeder F20.
In FIG. 11, distribution transformers, for example, columnar transformers are connected between the respective phases A, B, C and the ground, or between the respective phases from the section 102 to the section 105 and from the section 202 to the section 205. However, in the multiple direct grounding system, there are very many cases in which the distribution transformers are connected between the respective phases and the ground.
FIG. 12 is a circuit diagram showing the details of FIG. 11.
Referring to FIG. 12, the same references denote like parts in FIG. 11. References 101A, 101B and 101C denote CTs located in the respective phases, 30A, 30B and 30C are over-current relays, and 30N is a ground over-current relay.
Also, in the figure, references 50A, 50B and 50C denote the distribution transformers in the respective phases of the distribution, and 60A, 60B and 60C are loads connected to the respective phases.
The distribution transformers 50A, 50B and 50C and the loads 60A, 60B and 60C are connected between the respective phases of the distribution and the ground line 5, and there are very many cases in which the amount of load is different in the respective phases depending on the sections. There is an extreme case in which a load of only the phase A is connected.
In addition, in the respective instantaneous values, it is impossible that the three phases are balanced with each other but always unbalanced with each other, as a result of which a residual current flows in the neutral line. That is, a current also appears in a residual circuit at the secondary side of the CT 101, and the ground over-current relay 30N may malfunction depending on the magnitude of the current that flows in the residual current.
FIG. 13 is a circuit diagram showing a trip circuit of a conventional feed circuit breaker.
Referring to FIG. 13, reference P denotes (+) side of a control power supply, N is a (−) side of the control power supply, 51S is a contact of the over-current relays (30A, 30B, 30C) for short-circuiting protection, 51G is a contact of the ground over-current relay (30N), 52TC is a trip coil of the circuit breaker 100 or 200, and 52a is an auxiliary contact a of the circuit breaker 100 or 200.
Therefore, if the residual current reaches or exceeds the operation level of the ground over-current relay 30N due to an imbalance of the load, it is apparent that the ground over-current relay 30N malfunctions. The malfunction is caused by the apparent zero-phase current produced by the fourth line of the multiplex grounding system, that is, the neutral line and the imbalance of the loads of the respective phases. As a result, the circuit breaker 100 or 200 of the high-voltage distribution is opened by the malfunction of the ground over-current relay 30N even when no real ground fault occurs. Since the conventional distribution system protection is structured as described above, the ground protection cannot be surely conducted, and, in order to prevent the malfunction of the ground over-current relay 30N, the ground over-current relay must be artificially locked.
Since the conventional distribution system protection is structured as described above, the ground protection cannot be surely provided, and, in order to prevent a malfunction of the ground over-current relay 30N, the ground over-current relay must be artificially locked.